Where is a good place to find dragons to kill?

The best way to find a dragon is at one of the many shout word altars. They appear as dragon heads on your map and compass. The easiest one I can tell you about is near Ivarstead. Just go south from there, and you will eventually get a dragon head icon to show up on your compass. Just go there, and you will meet a dragon, and get a new word for a shout.

You could also try fast travelling around as this is often a good way to invoke a dragon attack. Just travel to a location and see if there's a dragon around the place. If there is, great. If not, just travel to another location and repeat.

See the list of all Dragon Lairs and their locations here (UESP Wiki). Note that (from the same link):

The dragons found at Dragon Lairs will always respawn, no matter how many times they are killed. This is an excellent way for any player with the skill to kill them to earn valuable experience and items and to test their combat prowess. It is especially helpful for lower to intermediate level players.

The radiant Companions quest, Dragon Seekers will have you killing dragons. This is also the case with one of the random radiant Bounty quests (a quest given by an innkeeper, jarl, or steward), and with the radiant Blades quest, Dragon Hunting.

Note that there is a bug regarding respawned dragons:

Respawned dragons found in previously cleared Lairs will not always give dragon souls.

This is fixed by the Unofficial Skyrim Patch (original / Legendary Edition - PC) / (Special Edition - PC) / (PS4) / (Xbox One) :

v1.0 - (2012-04-07): Fixed some encounter dragons not burning up nor giving the player a soul when slain (respawned dragons were not being properly reset)

The best way to consistently locate dragons is to complete the Companions quest line. After finishing the quests 'Glory of the Dead' and 'Dragon Rising' you can approach Farkas or Vilkas and ask for work. They will then give you the radient quest 'Dragon Seekers'. This quest randomly selects from a list of Dragon Lairs and sends you to kill a levelled dragon there. You will find some nice loot and the quest giver will give you a levelled sum of gold. Unfortunately there is a bug associated with this quest where the dragons will occassionally not grant a soul, but you'll still get some loot and it can be repeated as needed.

You can also locate dragons by fast travelling across the map randomly, as they will often appear after fast travelling to a location. You can hop around the map until you find a dragon to fight, then repeat as needed. Could take quite some time as this is random.

Best way to find them is going to high Hrothgar and asking for a word of power location. Following that will mostly result in a new dragon camp. If not, just go wander between towns and do any camp that shows. You'll find a dragon camp eventually and if you have a few you can occasionally travel between them to find normal dragons.

Go to Dawnstar. There is almost always a dragon attacking Dawnstar.